1. Field of the Invention
The invention is directed to method and apparatus for detecting fluctuations in an electromagnetic signal emitted by a receiver of electromagnetic radiation in response thereto, in the presence of background radiation. In particular, the invention is directed to methods and apparatus utilizing a beam of incident electromagnetic radiation having a spatial distribution of intensity periodically varying in time, and determining the correlation, if any, between the periodicity of the periodically varying spatial distribution of intensity and fluctuations in the measured signal emitted by a receiver of the incident beam. The invention has particular application for detecting the presence of a particle of relatively small dimensions, which elastically or inelastically scatters radiation in response to the incident radiation such as fluorescently labeled cellular particles in physiological fluids, which exhibit relatively low fluorescent fluctuations from the mean fluorescence of a sample, or such as light directly scattered by the particle which is weak compared to the mean scattering of a sample. The methods and apparatus may utilize modulation or periodic change of the spatial pattern of light intensity created by a coherent light source transmitted via a light guide to the sample being analyzed.
2. State of the Art
The use of laser beams and slits to differentiate particles based on their relative size by the correlation of fluorescent fluctuations in a relatively large sample volume is described in: Briggs, et al. "Homogeneous Fluorescent Immunoassay," Science, No. 212, 1266-1267 (1981) and Nicoli, et al., "Fluorescence Immunoassay Based on Long-Time Correlations of Number Fluctuations," Proc. Natl. Acad. Sci. U.S.A. 77(8), 4904-4908(1980).
The phenomena of light intensity distribution at the output plane of an optical fiber has been described by Rosen et al., "Speckle in Optical Fibers," Proceedings of the Society of the Photo-Optical Instrumentation Engineers, Volume 243, 28-33(1980); Kalnitskaya et al., Opt. Spectrosc.(USSR), 44(2), February (1978); Rosen et al., "Frequency Dependence of Modal Noise in Multi-Optical Fibers," J. Opt. Soc. Am., Vol. 70 No. 8 968-976, August (1980); and Kohler et al., "Speckle Reduction in Pulsed-Laser Photographs," Optics Communications, Vol. 12 No. 1, 24-28, September (1974). Additionally, U.S. Pat. No. 3,588,217, describes an apparatus for displaying an image of a hologram, utilizing a fiber optic device located to form part of the path of the coherent light illuminating the hologram which is moved at its mid-section while the ends are held rigid as the hologram is viewed to reduce the speckle noise that otherwise occurs in the hologram.